Magnetic resonance studies of tris-(8-hydroxyquinoline) aluminum-based organic light-emitting devices

Abstract

The electroluminescence (EL)-, electrical current density $(J)\ensuremath{-},$ and photoluminescence (PL)- detected magnetic resonance (ELDMR, EDMR, and PLDMR, respectively) of tris-(8-hydroxyquinoline) aluminum $({\mathrm{Alq}}_{3})$-based organic light-emitting devices (OLEDs) and ${\mathrm{Alq}}_{3}$ films is described. At low temperatures, a positive spin-1/2 resonance is observed, i.e., the changes in $J,$ the EL intensity ${I}_{\mathrm{EL}},$ and the PL intensity ${I}_{\mathrm{PL}}$ are positive $(\ensuremath{\Delta}J/J,$ $\ensuremath{\Delta}{I}_{\mathrm{EL}}{/I}_{\mathrm{EL}},$ and $\ensuremath{\Delta}{I}_{\mathrm{PL}}{/I}_{\mathrm{PL}}>0).$ $\ensuremath{\Delta}J/J$ and $\ensuremath{\Delta}{I}_{\mathrm{EL}}{/I}_{\mathrm{EL}}$ are insensitive to the nature of the ${\mathrm{Alq}}_{3}/\mathrm{cathode}$ interface. They weaken with increasing $T$ and become unobservable above 60 K. $\ensuremath{\Delta}{I}_{\mathrm{PL}}{/I}_{\mathrm{PL}}$ also decreases with $T,$ but is still observable at 250 K. Since the resonances all have the same $g$ value, similar linewidths, and a similar dependence on $T$ and the excitation level $(J$ or the laser power), they are all attributed to the same mechanism. That mechanism is either the reduction of singlet exciton (SE) quenching by a reduced population of polarons in the bulk of the ${\mathrm{Alq}}_{3}$ layer (``the quenching mechanism''), or the enhanced formation of SEs from singlet polaron pairs at the expense of triplet excitons (TEs) (``the delayed PL mechanism''). However, the latter mechanism implies that the yield of SEs in ${\mathrm{Alq}}_{3}$-based OLEDs is greater than 25%. Due to evidence to the contrary, and other evidence which is inconsistent with the delayed PL mechanism, we conclude that the positive spin-1/2 resonance is due to the quenching mechanism. At $T\ensuremath{\approx}60\mathrm{K},$ another spin-1/2 resonance, which reduces both $J$ and ${I}_{\mathrm{EL}}$ (but is unobservable in the PL), emerges and grows with increasing $T.$ This negative EDMR and ELDMR is sensitive to the buffer layer between ${\mathrm{Alq}}_{3}$ and the cathode, and is attributed to the magnetic resonance enhancement of the spin-dependent formation of negative spinless bipolarons from spin-1/2 negative polarons at the organic/cathode interface. The increased trapping of injected electrons at the interface reduces $J$ and consequently ${I}_{\mathrm{EL}}.$ However, at 295 K, the ratio $|\ensuremath{\Delta}{I}_{\mathrm{EL}}{/I}_{\mathrm{EL}}|$ in ${\mathrm{Alq}}_{3}{/\mathrm{A}\mathrm{l}\mathrm{O}}_{x}/\mathrm{Al}$ devices to that in ${\mathrm{Alq}}_{3}/\mathrm{C}\mathrm{s}\mathrm{F}/\mathrm{A}\mathrm{l}$ devices is significantly lower than the ratio of $|\ensuremath{\Delta}J/J|$ in these devices. Hence we suspect that other mechanisms, unidentified at this point, are also contributing to the negative ELDMR.